It is well known that semiconductor electronic devices comprising pressure sensors and optical sensors have become more important and are more frequently used. To access the active portion of the electronic devices that comprises the sensor, it is necessary to form packages that include a window.
FIG. 1 shows a perspective view of a package 1 for an electronic device with a sensor formed according to a first known method. Beginning from a substrate 2, for example, made from semiconductor material, dies made from semiconductor material are formed and comprise an electronic circuit 3. Such dies are encapsulated by a package 4 made from plastic material, which leaves a portion of the electronic circuit 3 uncovered. For example, an overmolding technique is used to form the package 4 such that a liquid epoxy resin 5 is dispensed on the substrate 2 and on the perimeter of the electronic circuit portion that must remain uncovered.
The first method is advantageous for many reasons, but it also has some drawbacks. In fact, this method does not produce regular surfaces like the ones formed using the traditional molding method. Moreover, the dimensions of these packages are not completely checkable and the resin depositing process is not easily repeatable, especially on the electronic circuit surface.
Indeed, a second method uses dams 6 that are placed on the substrate 2, as shown in the sectional view of a package in FIG. 2. Liquid epoxy resin is then dispensed between the dams, the substrate and the electronic circuit perimeter. Although this approach addresses the proposed objective, it presents some disadvantages. In fact, the resin outline on the integrated circuit surface cannot be maintained at a constant level during the production cycle.
Recently, methods using standard molding techniques for the production of semiconductor packages with a window have been introduced. FIG. 3 shows a sectional view of a mold 7 that comprises a half-mold 7a and a half-mold 7b, which define a space 7c for containing the electronic device 3 when they are laid on each other. According to this known technique, the upper half-mold 7b internally comprises a spring clamp 8 placed in a position almost corresponding to the portion of the electronic circuit to be left uncovered.
Such a spring clamp 8 basically has a parallelepiped shape and is slid into a recess in the upper half-mold 7b. The spring clamp 8 is biased by elastic elements, for example springs. When the half-mold 7b is laid on the half-mold 7a, the spring clamp 8 abuts against the device surface that is to be left empty. A high temperature molten epoxy resin is then injected in the mold to form the plastic package.
Although this method is advantageous for many reasons, it presents the drawback of damaging the circuit operation. In fact, the force applied on the two half-molds during the reciprocal coupling, to prevent the resin from pouring out of the molds when the injection occurs, is about 10-30 tons. Indeed, the force of the spring clamp abutting down against the electronic circuit does not have to be higher than 100 Kilos to possibly damage the electronic circuit 3. The presence of the spring clamp partially allows the force of the floating clamp abutting the electronic circuit to be modulated. Nevertheless, the direct contact of the metallic clamp against the active surface of the circuit can damage the functionality of the circuit, as shown in FIG. 4.
A further drawback occurs when the electronic circuit is tilted on the clamp surface, as shown in FIG. 5. In fact, the method shows an undesired resin infiltration during the mold-filling phase. Also, the electronic circuit surface that is not perfectly parallel to the support is damaged. It has been suggested to coat the mold and the spring clamp with a film tape 9, as shown in FIG. 6, to reduce the possibility of damaging the electronic circuit during, coupling of the two half-molds. Nevertheless, this method can correct only small irregularities of the integrated circuit. The film tape 9 must be also substituted at every new production phase, thereby increasing the production costs.
Further, the uniform deposition of the film tape on the half-mold is very critical. The mold used in conventional molding techniques provides a plurality of mold shapes having adjacent cavities to simultaneously form a plurality of packages. Therefore, the spring clamp must be introduced into the mold for each device to be molded. All of these inserts increase the possibility of having a part of the resin fill the spaces between the mobile portions during the mold-filling phase, thereby causing a jam or a yield reduction due to the resin being deposited on the integrated circuit surface.